Overload and short-circuit protection for voltage regulated power supply



Jan. 30, 1968 c, CONNOR 3,366,871

OVERLOAD AND SHORT-CIRCUIT PROTECTION FOR VOLTAGE REGULATED POWER SUPPLYFiled March 29, 1965 I a I INVENTOR.

0415 C Cami/0E Y fdwxg harm lfrhfarrley United States Patent 3,366,871OVERLGAD AND SHORT-CIRCUIT PROTECTION FOR VOLTAGE REGULATED POWER SUPELYDate C. Connor, Washington, la., assignor to Radio Corporation ofAmerica, a corporation of Delaware Filed Mar. 2% 1965, Ser. No. 443,2995 Claims. (Cl. 3239) ABSTRAT OF THE DISCLOSURE A voltage regulator isprovided which includes means for limiting the current fed to the loadsupplied thereby upon the occurrence of a short circuit or an overload,and which includes, in addition, an oscillator means to turn the voltageregulator off and on periodically during the duration of the shortcircuit or overload.

This invention relates to direct current voltage regulated powersupplies and more specifically to such power supplies which incorporateoverload and short-circuit protection.

When the output of a voltage regulated power supply is short-circuitedor overloaded, the voltage thereacross drops to zero or to below the setvoltage thereof, yet the power supply attempts to continue to supply itsoverloaded output with the voltage for which it is set. This actionresults in the power supply providing more current than its rated value,whereby components of the power supply can be damaged. If means areprovided for turning oif the power supply when its output isshortcircuited, it may be necessary for an operator to notice that thepower supply is off and to turn the power supply on again after theshort-circuit or overload has been cleared.

Voltage regulated power supplies are known which include means forturning off the power supply when their output circuits are overloadedand for turning the power supply on periodically thereafter to determineif the short-circuit still continues, and for causing the power supplyto continue to remain on when the short-circuit or overload is removed.However, such power supplies so equipped include oscillators which arecontinuously energized by sources of energy individual to theoscillators. If this individual source of energy should fail, theregulated power Supply will not be turned off responsive to its outputbeing short-circuited, whereby short-circuit protection of the powersupply will fail. Furthermore, since the components of the oscillatorcontinually carry current, the components continually wear, whereby theoscillator can fail due to failure of a component.

It is an object of this invention to provide an improved voltageregulated power supply including shortcircuit and overload protectionmeans.

It is a further object to provide an improved voltage regulated powersupply which turns itself off upon the occurrance of a short-circuit oran overload and which periodically checks for the continuing occurrenceof the overload and which turns itself on which the overload is removed.

It is still a further object to provide such an improved voltageregulated supply in which no additional or individual source of voltageis required to energize the means for periodically checking for thecontinuing occurrence of the overload.

It is another object of this invention to provide such an improvedvoltage regulated power supply in which no current flows through themeans for checking the continuance of the overload when the power supplyis operating normally.

It is an additional object to provide an improved voltage regulatedpower supply in which the series regulating transistor is protected fromthe occurrence of input over-voltage.

In accordance with this invention, upon occurrence of a short-circuit oroverload in the output of the voltage regulator, the excessive currentflowing in the output circuit causes the impedance of the seriesregulating impedance comprising a part of the voltage regulated supplyto be increased to a point where the output current is limited to apredetermined maximum, whereby the voltage drop across the seriesregulating impedence rises to a predetermined value. The voltageappearing across the series regulating impedance is caused to energizean oscillator, which causes blocking of the series regulating impedancefor the major portion of the cycle of operation of the oscillator andcauses unblocking of the series regulating impedance for the remainderof the cycle. The ratio of the on-to-olf time of the series regulatingimpedance is so chosen that the average energy absorbed in the seriesregulating impedance over the duration of the overload is below therated value of the dissipation for the series regulating impedance. Thecurrent limiting portion of the power supply continues to limit themaximum current through the series regulating impedance, whenconductive, to the predetermined, safe, maximum value. When the short oroverload is removed, the current limiting portion no longer causes theseries regulating impedance to exhibit a high impedance and the voltagesupply is removed from the oscillator, whereby, within the period of onecycle of operation of the oscillator, the voltage regulated power supplyoperates normally to provide voltage regulated current to the loadconnected to its output terminals. Since no voltage is applied to theoscillator except during shortcircuit or overload conditions of thepower supply, the oscillator elements carry no operating currents exceptduring such short-circuit or overload conditions, whereby the oscillatorelements have an extended life. Furthermore, since the voltage supply ofthe oscillator is that developed across the series regulating element,no additional or individual voltage supply is required for theoscillator. In addition, if the A.C. input voltage provided for thevoltage regulated supply goes so high that in normal operation of theregulated supply, the voltage drop across the series regulatingimpedance becomes sufficient to cause excessive dissipation andresultant injury to it, the voltage thereacross will energize theoscillator to cause it similarly to block the series regulatingimpedance, for the duration of the A.C. overvoltage, thereby protectingthe series regulating impedance.

The novel features of the invention, both as to its organization andmethod of operation, as well as additional objects and advantagesthereof, will be understood more readily from the following descriptionwhen read in conjunction with the single figure of the drawing which isa circuit diagram of an overload and short-circuit protected voltageregulated current supply constructed according to one embodiment of thisinvention.

An alternating current source (not shown) for the regulated supply 10 isconnected across the primary winding 12 of a transformer 14. The anodesof two rectifiers 16, 16 are respectively connected to the two terminalsof the secondary winding 18 of the transformer 14. The cathodes of theserectifiers 16 are connected together and to a first or positive terminalof a filter capacitor 20. The other or negative terminal of the filtercapacitor 20 is connected to the center tap of the secondary winding 18.The positive terminal of the capacitor 20 is connected to the collectorof an NPN transistor 22, the emitter of which is connected to thepositive output terminal 24 of the voltage regulated supply 11.8 by wayof the positive bus 26. All transistors illustrated in the figure are ofthe NPN type. The negative terminal of the capacitor 28 is alsoconnected to the negative terminal 28 of the supply through a currentsensing resistor 32, and the negative bus 34 in tandem. A filtercapacitor 36, in parallel with the series combination of two fixedresistors 38 and 40 and an intervening potentiometer 4-2 are connectedacross the terminals 24 and 28 of the regulated supply 10. The seriescombination of a resistor 44 and a further filter capacitor 46 is alsoconnected across the output terminals 24 and 28, a Zener diode 48 beingconnected in parallel with the filter capacitor 46, whereby, in theoperation of the supply 10, a reference voltage appears across the Zenerdiode 48.

Two other transistors 50 and 52 are connected with the series regulatingimpedance comprising the transistor 22. The collectors of the twotransistors 50 and 52 are each connected to the collector of the seriesregulating transistor 22. The emitter of the transistor 50 is connectedto the base of the transistor 22 and to the negative bus line 34 througha resistor 54. The emitter of the transistor 52 is connected to the baseof the transistor 50 and to the negative bus line 34 through a resistor56. The collector of the transistor 52 is also connected to its basethrough the two resistors 58 and 60 in series, and the junction 62 ofthe two resistors 58 and 60 is connected to the negative bus 34 throughthe parallel combination of a Zener diode 64 and a filter capacitor 66,which help to stabilize the voltage supplied to the resistor 60.

An amplifying and control transistor 68 is provided having its collectorconnected to the base of the transistor 52 and its emitter connected tothe junction of the Zener diode 48 and the capacitor 46. The base of thecontrol transistor 68 is connected to the slider of the potentiometer42.

The operation of the circuit so far described is as follows: anadjustable portion of the potential appearing between the outputterminals 24 and 28 of the voltage regulating supply 10 is applied tothe base of the transistor 68. This voltage is compared with a referencepotential appearing across the Zener diode 48. If the voltage on thebase of the transistor 68 is greater (more positive) than the voltabe onits emitter, the control transistor 68 becomes more conductive and thetransistors 52, 5t), and 22 all become less conductive, whereby thevoltage across the output terminals 24 and 28 tends to drop off. If thevoltage on the base of the control transistor 68 decreases with respectto the control potential applied to the emitter thereof, the transistors52, 50, and 22 become more conductive and the voltage across the outputterminals 24 and 28 tends to go up. The result is that the voltageacross the output terminals 24 and 28 remains at a steady valuedetermined by the setting of the potentiometer 42.

If a short-circuit or overload should occur across the output terminals24 and 28, then, in the circuit so far described, the transistors 52, 58and 22 would be made fully conductive in an attempt to raise the voltageapplied to the output terminals 24 and 28 to the value determined by thesetting of the potentiometer 42. During a shortcircuit or overload, thevoltage across the output terminals 24 and 28 is reduced, the currentflow through the series regulating transistor 22 would be high and thevoltage drop across it would be high whereby the energy absorbed by theseries regulating transistor 22 could be in excess of its maximum ratedvalue of dissipation which would cause damage thereto. Means areprovided to limit the current flow through the series regulatingtransistor 22 upon occurrence of a short-circuit or overload across theoutput terminals 24 and 28. This means includes the current sensingresistor 32.

A transistor 70 is provided whose emitter is connected to the negativeterminal of the filter capacitor 28. The base of the transistor 70 isconnected to its emitter through a smoothing capacitor 72, and this baseis also connected to the junction of the negative bus 34 and theresistor 32 through an additional resistor 74. The collector of thetransistor 70 is connected to the emitter of the control transistor 68.When the current flowing into the load (not shown) connected across theoutput terminals 24 and 28 tends to be excessive, the voltage dropacross the resistor 32 rises to the point where the transistor 78 ismade conductive, applying a shunt across the Zener diode 48, and thereference potential appearing across the Zener diode 48 drops. When thereference potential drops, the control transistor 68 causes the seriesregulating transistor 22 to increase in impedance to maintain thecurrent therethrough at a constant maximum value. Due to the non-linearcurrent v. voltage characteristic of the transistor 70, the transistor 70 acts as a threshold device, and has no effect on the voltage regulatedsupply until the current supplied thereby reaches its rated value, butthen limits the maximum current supplied by the source 10 to a smallamount over its rated value. However, with a high voltage drop acrossthe series regulating transistor 22 (due to its impedance beingincreased) and with a high, limited, current flowing therethrough, aheavy overload is applied to the series regulating transistor 22, whichmay thereby be damaged. Therefore, in the absence of the furtherapparatus to be described, when a shortcircuit or overload is applied tothe voltage regulated supply 10, the series regulating impedance 22 isoverloaded for the duration of the short-circuit or overload.

Further means are provided to protect the series regulating impedance 22for the duration of the short-circuit or overload. The protective meansfor the series regulating impedance 22 comprises an oscillator 76, meansfor energizing it, and means for coupling the oscillations producedthereby to the series regulating impedance 22.

The oscillator 76 may be a multivibrator comprising two transistors '78and 88. The base of the transistor 78 is connected to the collector ofthe transistor 80 through a parallelly connected RC circuit 82 and thebase of the transistor 80 is connected to the collector of thetransistor 78 through a second parallelly connected RC circuit 84. Thecollectors of the transistors 78 and 80 are connected together throughrespective resistors 86 and 88.

The multivibrator 76 is energized by a connection from the positiveterminal of the filter capacitor 20 through a Zener diode 90 and acurrent limiting resistor 92 connected in series to the junction 91 ofthe resistors 86 and 88, and from the emitter of the series regulatingtransistor 22 to the emitters of the transistors 78 and 80 throughindividual load resistors 93 and 92. The load resistor 93 may comprise afixed and variable portion in series as shown. The junction 91 is alsoconnected to the emitter of the series regulating transistor 22 througha protective Zener diode 94 which acts to protect the oscillator 76 fromover voltages.

The junction of the variable and the fixed portions of the resistor 93is connected to the base of a transistor 96. The emitter of thetransistor 96 is connected directly to the emitter of the seriesregulating transistor 22. The collector of the transistor 96 isconnected to the collector of the control transistor 68.

A signal lamp 98 and a current limiting resitsor 100 are connected inseries across the main electrodes comprising the collector and emitterof the series regulating transistor 22 for a purpose to be described.

When the series regulating transistor 22 is conductive in a normalmanner, the voltage drop thereacross is insufficient to cause noticeableillumination of the signal lamp 98, and this voltage is too low to breakdown the Zener diode 90. Therefore, little or no current flows throughthe signal lamp 98, and no current flows through the Zener diode 90 whenthe regulator is supplying voltage regulated current in a normal mannerto its output terminals 24 and 28, and therefore no voltage is appliedto the oscillator 76 and no current flows through its components. When,due to excessively high current flowing in the current sensing resistor32, the impedance of the series regulating transitsor 22 is raised tothe point of limiting the output current at a value just above the ratedoutput of the voltage regulated source (as discussed above), the voltageacross the series regulating transistor 22 goes up to the point wherethe signal lamp 98 is illuminated, indicating the presence of anoverload or short-circuit. Also the Zener diode 9t) breaks down, wherebya voltage is applied across the several circuit components comprisingthe oscillator 76, causing it to oscillate; A portion of the oscillationoutput of the oscillator 76, appearing at the emitter of the transistor78, is applied between 'the base and emitter of the transistor 96 by thevariable portion of the resistor 93 to render the transistor 96conductive and non-conductive alternately, and thereby to render theseries transistor 22non-con-ductive and conductive alternately. Thefrequency of the oscillator 76 may be about one cycle per second, andits duty cycle, as applied to the transistor 96, may be about 2 and ahalf percent, whereby the series transistor 22 is rendered conductive tosupply current to the output terminals in short evenly spaced pulses forabout 2 and a half percent of the time that the output remainsshort-circuited or overloaded. No damage occurs in the series regulatingtransistor 22 during shortcircuit or overload conditions since theaverage current flow in the transistor 22 is below the rated maximumvalue thereof. During the time that the transistor 22 is made conductiveby the oscillator '76, that is during the time the transistor 78 isnon-conductive, the current through the transistor 22 is limited, asnoted above, by action of the transistor '70. During this conduction ofthe transistor 22, the voltage thereacross goes down, whereby theintensity of the light 98 decreases but it does not go off. The voltageacross the transistor 22 is still suiiicient to break down the Zenerdiode 9t and the oscillator 76 is still energized. The periodic changeof intensity of the light 98 indicates that the oscillator 76 isoperating to turn the transistor on for a short interval in spite of thecontinuing short-circuit. During the period of continuing short-circuitor overload, the voltage across the transistor 22 is such that theoscillator 76 continues to oscillate. When the short-circuit or overloadis cleared, the voltage across the load will go up and the voltageacross the transistor 22 will go down and the light 98 will go off. Dueto the drop of voltage across the transistor 22, the Zener diode 90 doesnot pass current and no voltage will be applied to the oscillator 76.The current flow through the transistor 22 reduces to the rated valuethereof or lower and the transistor 70 becomes non-conductive. Thevoltage applied to the base of the transistor 22 is then determined bythe setting of the potentiometer, that is, the regulated supply actsnormally.

If the voltage of the A.C. supply (not shown), connected to the primarywin-ding 12 increases to the point where the voltage across the seriesregulating transistor 22 is sufiicient to light the signal lamp 98 andto break down the Zener diode 90, the oscillator 76 causes blocking ofthe transistor 22 during about 97 and a half percent of the duration of.the excessively high input voltage. Therefore, during the occurrence ofexcessive high input voltage to the supply 10, the series regulatingimpedance 22 thereof is protected from damage due to excessivedissipation. When the A.C. supply voltage drops to its normal value, thevoltage across the transistor 22 also drops to the point where the Zenerdiode 90 is not broken down and (in the absence of an overload orshort-circuit) the voltage regulated source 10 operates in a normalmanner.

Although only a single overload and short-circuit protected voltageregulated supply has been described, variations are possible within thespirit of the present invention. For example, while all transistors havebeen shown and described as being NPN type, by appropriate reversal ofpotentials or of circuit connections, PNP type may be used. If theseries regulating transistor 22 has sufiicient current multiplication,the junction of the collectors of the transistors 68 and 96 may beconnected directly to the base of the series transistor 22, in whichcase the transistors 50 and 52 and the resistors 54 and 56 connectedthereto may be omitted. Hence it should be understood that the foregoingdescription is to be considered as illustrative and not in a limitingsense.

What is claimed is:

1. An overload and short-circuit protected voltage regulation powersupply comprising:

a pair of input terminals to be connected to a source of unregulateddirect voltage,

a pair of output terminals for said supply,

respective connections between said input and output terminals includinga series regulating impedance between one of said pair of inputterminals and one of said pair of Output terminals, and a currentsensing resistor between an input and an output terminal,

a reference voltage means,

means for comparing at least a portion of the output voltage appearingacross said output terminals with the reference potential across saidreference voltage means,

means responsive to said comparing means for varying the impedance ofsaid series regulating impedance in a sense to maintain the voltageacross said output terminals at a constant value, and

means responsive to a predetermined current flowing through said currentsensing resistor for decreasing the voltage provided by said referencevoltage means, whereby the current supply by said regulated supply islimited to a maximum value.

2. An overload and short-circuit protected regulated direct currentsupply comprising:

a pair of input terminals to which an unregulated direct current may beapplied,

a pair of output terminals,

respective connections between said input and said output terminals,

said connections including a series regulating impedance and a currentsensing resistor,

means for varying the impedance of said series regulating impedance inresponse to a variation in the voltage across said output terminals,

means responsive to a predetermined flow of current in said currentsensing resistor to increase the impedance of said series regulatingimpedance, whereby the voltage across said series regulating impedancemcreases,

an oscillator and a voltage threshold device connected in series acrosssaid series regulating impedance, said oscillator producing oscillationswhen the voltage threshold of said voltage threshold device is exceeded,and

means for applying said oscillations to said series regulating impedanceto render it alternately conductive and nonconductive whereby theaverage energy dissipation in said series regulating impedance isreduced.

3. An overload and short-circuit protected voltage regulated directcurrent supply comprising:

a pair of input terminals to which an unregulated direct current sourcemay be connected,

a pair of output terminals,

a transistor having a pair of main electrodes and a control electrode,

a connection between one of said input terminals and one of said outputterminals including said main electrodes,

a connection between the other of said input and output terminals,

one of said connections including a current sensing resistor,

a potentiometer connected across said output terminals.

a resistor and a voltage threshold device connected across said outputterminals,

a second transistor having a pair of main electrodes and a controlelectrode,

a connection between said control electrode of said second transistorand the slider of said potentiometer,

a connection between one of the main electrodes of said secondtransistor and the junction of said resistor and said threshold device,

a connection between the other main electrode of said second transistorand the control electrode of said first transistor,

a third transistor having a pair of main electrodes connected acrosssaid voltage threshold device and a control electrode,

connections for applying the voltage developed across said currentsensing resistor between the control electrode and one of the mainelectrodes of said third transistor,

a second voltage threshold device and an oscillator connected in seriesacross the main electrodes of said first transistor, and

connections for applying the oscillations produced by said oscillatorbetween the control electrode and .a main electrode of said firsttransistor.

4. A regulated direct current supply comprising:

a pair of input terminals to which an unregulated direct current sourcemay be connected,

a pair of output terminals,

a transistor having a pair of main electrodes and a control electrode,

a connection between one of said input terminals and one of said outputterminals including said main electrodes,

a connection between the other of said input and output terminals,

' a voltage threshold device and an oscillator connected in seriesacross the main electrodes of said transistor, and

connections for applying the oscillations produced by said oscillatorbetween the control electrode and a main electrode of said transistor.

5. An overload and short-circuit voltage regulated direct current supplycomprising:

a pair of input terminals to which an unregulated direct current sourcemay be connected,

a pair of output terminals,

a transistor having a pair of main electrodes and a control electrode,

a connection between one of said input terminals and one of said outputterminals including said main electrodes,

a connection between the others of said input and output terminals,

one of said connections including a current sensing resistor,

a second transistor having a pair of main electrodes and a controlelectrode,

connections for applying the voltage developed across said currentsensing resistor between the control electrode and one of the mainelectrodes of said second transistor,

connection means between a main elctrode of said second transistor andthe control electrode of said first transistor to apply a currentlimiting current to the control electrode of said first transistor uponoccurrence of a current of predetermined magnitude in said currentsensing resistor, whereby the voltage across said first transistorincreases,

a voltage threshold device and an oscillator connected in series acrossthe main electrodes of said first transistor, and

connections for applying the oscillations produced by said oscillatorupon breakdown of said threshold device between the control electrodeand a main electrode of said first transistor to render it alternatelynonconductive and conductive.

References Cited UNITED STATES PATENTS JOHN F. COUCH, Primary Examiner.

A. D. PELLINEN, Assistant Examiner.

